They can be stuborn little sods especially if they havent been appart for a long time ! a good dose of wd40 down where the tapered bit goes through the spindal ! and a bit of heat ( being carefull not to melt anything ) and a few good wallops and it usually goes .
Ive always found those fork style seperators hard work ! if you can get hold of one that latches over the joint and you tighten it up with a allen key or spanner and it pulls them appart ! always found those style seperators much better !
keep at it it will go eventually